Vitamin D plays a significant role in bone health by helping your body absorb calcium, and it also contributes to muscle strength, healthy immunity and the regulation of calcium levels in the bloodstream. The two primary types of vitamin D are vitamin D-2, which is made by plants, and vitamin D-3, which your body makes after exposure to sunlight. Getting enough vitamin D is crucial, but too much can pose significant health risks, as well. For this reason, combining a multivitamin and an additional vitamin supplement may not be necessary or recommended.
Recommended Intakes
To ensure adequate calcium absorption and meet your body's need for vitamin D, the Institute of Medicine recommends that adults consume 15 mcg daily. After the age of 70, the recommendation increases to 20 mcg daily. While exposure to sunlight for 10 to 15 minutes several times a week can help you meet your vitamin D needs, a limited number of foods -- including mackerel, salmon, tuna, sardines and beef liver -- are natural sources of vitamin D. Other foods may be fortified with either vitamin D-2 or D-3, including milk, orange juice, soy milk, cereal and yogurt.
Supplementation
A typical vitamin D supplement provides around 400 international units, or IU, of vitamin D-3, according to the Linus Pauling Institute at Oregon State University. That's the equivalent of 10 mcg. Some vitamin D supplements may contain as much as 2,000 IU. In multivitamins, the typical vitamin D dosage is 400 IU, or 10 mcg. Combining a multivitamin and a single-ingredient vitamin D supplement, then, would provide a total of 20 mcg or more of the nutrient. This dosage provides more than the RDA for adults under 70 years of age. As a general guideline, it's best to choose supplements that provide no more then 100 percent of the recommended dietary allowance.
Vitamin D Toxicity
Too much vitamin D can cause nausea, reduced appetite, weakness, constipation, kidney damage and weight loss, according to the National Institutes of Health Office of Dietary Supplements. Additionally, vitamin D toxicity raises calcium levels, which may lead to kidney stones, bone loss and eventually the calcification of vital organs, such as the kidneys and heart. The Institute of Medicine establishes upper tolerable intake levels for certain nutrients to indicate the highest dosage not likely to produce toxicity. The upper tolerable intake for vitamin D is set at 100 mcg for individuals age 9 and older.
Considerations
The risk for vitamin D deficiency is greatest when an individual has limited exposure to sunlight or when intake is lower than the recommended amount for an extended period of time. Risk is also greater if a condition is present that keep the kidneys from converting vitamin D to its active form or if absorption is inhibited. Signs of deficiency include muscle weakness and pain. Long-term deficiency can lead to rickets in children and osteomalacia in adults. Both conditions involve the deformation or softening of bones. Your doctor may recommend that you take more the the RDA of vitamin D to overcome deficiency. Follow your doctor's advice about supplementation and dosage.
References
- National Cancer Institute: Vitamin D and Cancer Prevention: Strengths and Limits of the Evidence
- Institute of Medicine: Dietary Reference Intakes (DRIs): Recommended Dietary Allowances and Adequate Intakes, Vitamins
- Linus Pauling Institute: Vitamin D
- Office of Dietary Supplements: Dietary Supplement Fact Sheet: Vitamin D
- Institute of Medicine: Dietary Reference Intakes (RDIs): Tolerable Upper Level Intakes, Vitamins
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